1. Technical Field
The present invention describes an integratable planar waveguide-type non-reciprocal polarization rotator. As an embodiment of the present invention, the planar waveguide-type non-reciprocal 90-degree polarization rotator includes optical waveguide-type input and output ports; a reciprocal 45-degree polarization rotator of an asymmetric optical waveguide structure; an optical waveguide-type non-reciprocal 45-degree polarization rotator with cladding layer of magneto-optic material; and a phase compensator which compensates the phase difference between polarizations by having positioned between the above reciprocal 45-degree polarization rotator and non-reciprocal 45-degree polarization rotator.
2. Description of the Related Art
On-chip-type non-reciprocal polarization rotators are important optical devices for future applications to integrated optical isolators and circulators and to various polarization sensors.
The non-reciprocal polarization rotator is an optical device rotating the polarization of an optical beam traveling in one direction by 90 degrees, but passing another optical beam traveling in the opposition direction without changing its polarization. The non-reciprocal polarization rotator can be used for applications to optical isolators and optical circulators by having polarization filters or polarization beam splitters combined at its input and output ports.
The optical isolators based on the non-reciprocal polarization rotator utilizing Faraday rotation function of the magneto-optic effect in bulk-optics have been used popularly. However, various approaches for integrated planar waveguide-type non-reciprocal polarization rotators, which can be integrated with other photonic devices, are still under development and not ready for a practical optical isolator of integration-type.
The conventional art of US 2013/0142475 describes an integrated non-reciprocal polarization rotator and an integrated optical isolator utilizing the integrated reciprocal and non-reciprocal polarization rotators, which have a block of magneto-optic material, such as bismuth europium holmium gallium iron garnet or bismuth yttrium iron garnet, located between two silicon waveguides, an index-matching layer formed between the silicon waveguide and magneto-optic material, and a magnetic field applied to the magneto-optic material by attaching a magnet. This prior art uses a scheme of 45° polarization rotation in the reciprocal polarization rotator section and additional 45° polarization rotation in the non-reciprocal polarization rotator section. However, this art requires a difficult fabrication process to form an index-matching layer between the silicon waveguide and magneto-optic material, and has a significant drawback of a high optical loss over the entire device.
Another prior art of the non-reciprocal polarization rotator has been demonstrated by using birefringence between two orthogonal polarization modes in an InGaAsP optical waveguide of asymmetric square shape with one side of inclined surface, which includes a hybrid integration of Ce:YIG crystal formed on the top of the waveguide and a magnetic field applied in an orthogonal direction to the light propagation direction [IEEE J. Quantum Electronics 46(11), 1662(2010)]. In this art, the etching control is not easy to form the asymmetric InGaAsP optical waveguide and a uniform bonding process of the magneto-optic crystal is difficult. Thus, this art has a drawback of low efficiency of the Faraday polarization rotation.
A prior art of a reciprocal polarization rotator describes reciprocal polarization rotation in a GaInAsP or Si waveguide having a long asymmetric trench pattered inside the waveguide [Opt. Express 17(14), 11267 (2009) & Opt. Commun. 324, 22 (2014)]. In this art, a long asymmetric trench is formed in the semiconductor waveguide, and the waveguide rotates the TE polarization mode into TM polarization mode for an optical beam travelling in either direction. However, this art cannot provide the function of non-reciprocal polarization rotation.
Another prior art of the reciprocal polarization rotator is proposed by numerical simulation on reciprocal 90° polarization rotation in a silicon nanowire waveguide with a partially etched section [J. Opt. Soc. Am. B 25(5), 747 (2008)]. This prior art describes only a scheme of reciprocal polarization rotation for optical beams in both directions, but cannot provide the function of non-reciprocal polarization rotation.